Nonhuman primates of the species Erythrocebus patas (patas), Macaca mulatta (rhesus), Macaca fascicularis (cynomologus) and Cebus apella (cebus), are subjected to direct-acting and metabolism-dependent chemical carcinogens by transplacental or direct exposure. In some cases the carcinogen-treated animals are subsequently exposed to chemicals that promote the development of neoplasms in rodents. Mechanisms of organ and species differences in the effects of chemical carcinogens and tumor promoters among rodent and nonhuman primate species are investigated. DNA repair capacity and its inducibility in different tissues are being surveyed in this context. Induced tumors are evaluated by light microscopy using standard staining procedures, histochemical techniques and electron microscopy and are assayed for in vitro cultivability and transplantability to rodents. Phenobarbital has been shown to promote development of hepatocellular neoplasms in patas monkeys after initiation by transplacental or direct exposure to nitrosodiethylamine even when as long as 4 years -- the duration of infancy and adolescence in this species -- elapsed between initiation and inception of promotion. This is the first demonstration of tumor promotion in a nonhuman primate species.